This invention relates to a muffler for an internal combustion engine such as a small air-cooled two-stroke gasoline engine which is suited for use in a portable small working machine such as a bush cutter or a chain saw, and, more particularly, to a muffler for preventing sparks from being discharged from the internal combustion engine and for minimizing final exhaust gas temperature.
In view of increasing environmental concerns with combustion products, reduced amounts of HC, CO, NOx and the like in engine exhaust gas are now mandated even for internal combustion engines such as small air-cooled two-stroke gasoline engines for use in portable small working machines such as bush cutters and chain saws. For example, according to an exhaust gas control bill in California, known as CARB TIER 2, limits on these noxious gases are becoming progressively more restrictive year by year starting in 1998.
In view of such exhaust gas regulations, the muffler disclosed in Japanese Patent Application H8-84260 has an expansion chamber into which exhaust gas emitted from the engine exhaust port is introduced. The expansion chamber is perpendicularly divided by a partition plate into a first expansion chamber and a second expansion chamber. The partition plate is provided with an exhaust emission purifier such as an oxidation catalyst in the form of a gas-permeable metallic foamed body. Exhaust gas introduced into the first expansion chamber then passes via the exhaust emission purifier to the second expansion chamber.
The Japanese Patent Application H8-84260 also discloses a muffler wherein, additionally, the outer wall of the expansion chamber is formed of a double wall consisting of an inner panel and an outer panel which are spaced apart by a suitable distance so as to form an air space therebetween.
The muffler disclosed in the Japanese Patent Application H8-84260 has been found to be highly effective in reducing THC components in an exhaust gas. However, after passing through the oxidation catalyst, hot exhaust gas is discharged immediately and directly from the second expansion chamber via a tail pipe of the muffler to the ambient, and there is a risk of live sparks being discharged to the ambient, and of the final exhaust gas temperature exceeding the upper limit mandated by the aforementioned regulation.